This invention relates to a coating composition, particularly for plastic substrates, and preparation and use thereof. More particularly this invention relates to a coating composition for forming a protective coating which provides excellent surface properties such as hardness, wear resistance and scratch resistance on the surfaces of shaped articles particularly made from plastics, a process for preparing said composition, a method for coating the surfaces of shaped articles particularly made from plastics, and the shaped articles thus coated.
Plastic shaped articles are widely used because of their advantages in good processability, light weight, impact resistance, and the like, but the range of their use is limited due to relatively low surface hardness and easiness in being scratched. Particularly scratches are conspicuous in plastic shaped articles having good transparency, so that improvement of this defect has strongly been desired.
In order to improve such a defect, various coating compositions and methods for coating these coating compositions have been proposed. For example, there have been disclosed (1) a method for coating hydrolyzed products obtained from methyltrialkoxysilane and phenyltrialkoxysilane (U.S. Pat. No. 3,451,838L ), (2) a method for coating a composition comprising a hydrolyzed product obtained from ethyl silicate and poly(vinyl acetate) resin (U.S. Pat. No. 2,404,357), (3) a method for coating a composition comprising a hydrolyzed product obtained from tetraalkoxysilane, a hydrolyzed product obtained from alkyl trialkoxysilane, and an alkali metal salt of organic carboxylic acid (U.S. Pat. No. 3,894,881), and the like. But these methods have respective defects and give no fully satisfactory results in practical use. The method (1) has defects in that the coating solution has short pot life, handling of the coating solution is difficult for the producer and there is insufficiently in hardness. The method (2) has defects in that coated film is poor in water resistance and hardness, it is practically very difficult to select a composition having good properties, since if the proportion of the hydrolyzed ethyl silicate in the composition of hydrolyzed ethyl silicate and poly(vinyl acetate) becomes large in order to increase hardness, cracks are easily produced with the lapse of time, and if the proportion of the poly(vinyl acetate) becomes large in order to prevent the production of cracks, water resistance becomes poor and hardness becomes insufficient. The method (3) has a defect in that there is a tendency to produce micro-cracks on coated film. Other methods for forming hard coating films on the surfaces of plastic shaped articles by using organic silicon compounds and curing with heating have also been proposed, but these methods have at least one defect among the following defects, i.e. it requires a high temperature and a long period of time for curing, pot life of a coating solution is short, coated film becomes white, coated film has poor water resistance, coated film has poor adhesion to a substrate, there is a tendency to generate cracks with the lapse of time (the most unpreferable defect), and the like. Therefore, there has been desired a composition for coating plastic shaped articles overcoming these defects mentioned above.